set $cookie_abc "$cookie_abc";

[amodpandey]

Thank you. But this is not what I am looking for. The question is about
defining $cookie_abc and default it to $cookie_abc itself (if the value
exists).

the map will give me the value in $abc which I am not looking for. 

BTW set $cookie_abc "$abc"; will still reset the value of $abc to default

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,236075,236085#msg-236085